1. Field of the Invention
The present invention relates to a method and a system for measuring critical dimension and monitoring fabrication uniformity, and more particularly to a method and a system for measuring critical dimension and monitoring fabrication uniformity from a scanned image with lower resolution.
2. Description of the Prior Art
Uniformity of semiconductor fabrication, which relates to manufacturing yield, can be obtained from critical dimension (CD) distribution in an entire wafer. Therefore, critical dimension is a significant feature in semiconductor manufacturing process. In recent years, line width, being a type of critical dimension and continuously shrunk with semiconductor process progress, is increasingly harder to be measured consequently.
Traditionally, CD-scanning electron microscope (CD-SEM) is used for measuring line width of a die. In order to provide scanned images with ultra high resolution, the probe spot of CD-SEM must be extremely small. Therefore, the electron current, also known as primary beam, provided by the CD-SEM is bound to be very small to reduce coulomb repel force between the electrons. However, in order to image, enough signal electrons, also known as secondary electron, must be collected by the detector of CD-SEM, so the scan speed must be extremely slow. In current CD metrology, only five dies on a 12″ wafer at 65 nm node are selected to be measured for critical dimension to represent fabrication uniformity of the entire wafer, because the inspection speed of the CD-SEM limits the number of dies can be inspected under the requirement of semiconductor manufacturing speed. However, while node proceeds to less than 32 nm on an 18″ wafer, not only critical dimensions of five dies can not represent the entire 18″ wafer any more, but the five dies will incur more inspection time.
Accordingly, how to more quickly measure critical dimension and monitor fabrication uniformity is now highly desirable.